Ants and Pests of Cocoa

The entry to the literature here has to be Entwistle (1972). It,
however, dealt with the pests of cocoa on a global basis and, although
a whole chapter is assigned to ants, predates almost all the work on
mosaics.

Interestingly, additional early information was collated by Bequaert
who wrote on "Ants in their diverse relations to the natural world".
His long list of references shows that, prior to 1922, there was only a
modicum of information from West Africa (Part IV in Wheeler, 1922). For
instance, there was work by Winkler (1905) in Cameroun, when it was
found that Crematogaster kohli (as africana variety winkleri)
gnawed the skins of cocoa pods, and Camponotus acvapimensis and
Oecophylla longinoda were "accused of the same evil"; Camponotus
brutus reportedly gnaws the base of "fruit-stalks" and licks up the
sap, an effect of this was to cause the fruits to dry up or drop off
(presumably the fruits were at the Cherelle stage). Winkler also
reported the involvement of ants in the pollination of cocoa. Lamborn
(1914a) wrote on trophobiosis between ants and coccids, membracids,
jassids and psyllids in southern Nigeria; including a report of Camponotus
acvapimensis (as akwapimensis variety poultoni)
tending a membracid. In Zaïre, Bequaert himself had observed Pheidole
punctulata with treehoppers and Myrmicaria eumenoides with
membracids. The habit of tent or cowshed building by ants over coccids,
etc., Bequaert noted as having first been described by P. Huber in 1810
for Lasius niger in Europe. An early English-language reference
with "tent-building ants" in its title was given by Bequaert as Couper,
W. (1863) Proc. Ent. Soc. Philadelphia, 1, 373-4.
Incidentally, for anyone wishing to study the subject, Bequaert's list
of references would serve as a very good source list of the early
literature.

The collecting efforts of Prof. F. Silvestri, which yielded so many
new species and new geographical records, came according to his report
title from an "Expedition to Africa in search of natural enemies of
fruit flies" (Silvestri, 1914, cited by Wheeler, 1922).

Ants and mirids, or capsids

Collingwood (1972, abstract only) reported that an analysis of
samples of 100 individual trees from each of 64 cocoa farm plots in
Ghana showed that both Oecophylla longinoda and Tetramorium
aculeatum (as Macromischoides aculeatus) were significantly
negatively associated with fresh capsid damage. Other dominant ants
were either weakly negatively associated or, as in the case of the
majority of Crematogaster species, positively associated with
damage, i.e. protective of capsids. A further analysis showed that the
net effect of 16.5% reduction in capsid damage was due to an
ant-negative association with Distantiella theobroma but not
with Sahlbergella singularis. From a large number of
samples (presumably a different set) using both pkd and direct trunk
counting on farm plots scattered throughout the cocoa growing area of
Ghana, and using the relativities he had derived from the first
exercise, it was calculated that the mean capsid reducing effect of the
presence of ants, mainly Oecophylla longinoda and Tetramorium
aculeatum was 15%. Therefore, on average over 80% of Ghana cocoa
was not protected from capsid injury by ants.

Ants and Homopterans

Strickland (1951a, b) showed how most Homopterans have a
close relation, perhaps dependency, with ants of one species or other.
The dependency however has a high degree of mutualism, as the sap
exudates of the Homopterans is a very useful source of energy for many
ant species. For most of the ant species regarded as dominants that
energy source is vital for the development and maintenance of their
large colonies. Indeed, perhaps the only dominant which does not rely
on Homopterans for energy is Tetramorium aculeatum.

Campbell (1994) provided a very useful
list. Although his work was in Ghana, and dealt with only Crematogaster
clariventris, Pheidole megacephala types (used as a
convenient grouping for several morphologically and behaviourally
similar taxa), and Tetramorium aculeatum, the list is
applicable to Nigeria and elsewhere in West Africa.

Aphids - Toxoptera aurantii (Boyer de Fonscolombe)
- associated with Pheidole megacephala and some Crematogaster
spp. but Campbell did not do any detailed studies.

Mealy bugs - Pseudococcidae - frequency from Campbell
(1994):

Planococcoides njalensis (Laing) - 47% - positively
associated with Pheidole megacephala and negatively with Tetramorium
aculeatum (although it will attend then if other ants absent).
Apparently Crematogaster africana will attend this species on Canthium
but not on cocoa.

Hard scales - Stictococcidae - Stictococcus sjostedti
Cockerell - the most abundant Homopteran, mostly attended by and highly
positively associated with Crematogaster clariventris, a
few tended by Pheidole megacephala, negatively associated with Tetramorium
aculeatum (although it will attend them if other ants absent).

Jumping plant lice - Psyllidae - Mesohomotoma tessmanni
(Aulmann) - as abundant as the stictococcids but apparently not
attended by ants. It, however, was most abundant on trees occupied by
more than one of the dominant ants, perhaps because there was less
well-organised ant activity in such circumstances.

Campbell (1994) supported the suggestion by Bigger (1993) that the
absence of honeydew exuding Homoptera is a key factor which allows Tetramorium aculeatum
to occupy trees as sole dominant; an associated factor was felt to be
the fact that Homoptera were absent from poorly growing trees, which
could account for the association between Tetramorium aculeatum
and shade. The observations at CRIN in Nigeria (Taylor & Griffin,
1981), during the 1975 study of the black pod disease epidemic, also
showed that densely planted cocoa had poor productivity, with few pods,
few Homoptera, and, in that instance (Block
E5/1), the canopy dominant was Tetramorium aculeatum.